1. Field of the Invention
This invention is concerned with forming of metals, by bending, for example, which induces tensile strain in some areas of the surface of the metal.
2. Description of the Related Art
Numerous useful parts and structures are fabricated from aluminum and aluminum alloys, which are malleable and exhibit good oxidation resistance and high strength-to-weight ratios. Thin-section aluminum materials are typically bent or formed to produce desired structures. For thick-section materials, however, cracks tend to form in surfaces undergoing tensile strain during bending or forming operations. Such cracks tend to propagate through the bulk material, resulting in premature failure. Bending and forming of thick aluminum materials is restricted to low bend angles and often requires use of elevated temperatures. Consequently, thick-section aluminum structures are typically fabricated by fusion welding together aluminum parts, usually in the form of plates. Other metals, titanium and ferrous materials, for example, also tend to develop surface cracks during thick-section bending or forming so that fusion welding is generally used to fabricate thick metal structures.
Build up of metal structures by fusion welding is inefficient and expensive compared to fabrication by bending or forming. In addition, fusion welds are prone to defects and tend to be relatively weak and have low ductility because of high residual stress, a coarse microstructure (similar to that of cast parts), weld defects, and extensive precipitate overaging in the heat affected zone. An effective method for bending and forming thick metals would reduce costs, enhance design flexibility, and improve the mechanical properties of thick metallic structures.
Friction stir processing (FSP) involves passing a rotating FSP tool through a metallic material to locally create a fine-grain microstructure providing improved mechanical properties [F. D. Nicholas, Advanced Materials Processes 6/99, 69 (1999)]. The FSP operation is typically performed at room temperature but the friction and metal deformation involved raises the local temperature to just below the solidus temperature so that the friction stir processed material is annealed and fully recrystallized. Since the material does not undergo melting during FSP, such as occurs during fusion welding, overaging in the heat affected zone is significantly less. Friction stir processing has been demonstrated for a variety of metals and alloys, including aluminum, titanium, bronze, and steel materials. The FSP approach has been used to locally improve the mechanical properties in high-stress areas of cast metal parts but has not previously been applied to enable bending of thick-section parts.